Phenols

FIELDS OF STUDY: Organic Chemistry

ABSTRACT

The characteristic properties and reactions of phenols are discussed. Phenols are limited in type because of their molecular structure, which also endows them with chemical reactivity. Phenols and phenolic compounds form an important class of naturally occurring compounds as well as industrially important materials.

The Nature of Phenols

Phenol, or hydroxybenzene, is the simplest of the class of compounds known as phenols and related phenolic compounds. Structurally, phenol consists of a ring-shaped benzene molecule in which one of the hydrogen atoms has been replaced by a hydroxyl group (−OH). The phenols are often referred to as aryl alcohols or aromatic alcohols. They are characterized by the presence of a hydroxyl group bonded to an aryl group (a broad class of ring-shaped functional groups that includes phenyl groups), and although the number of atoms that makes up the benzene ring is small, the class of phenols and phenolics is quite large.

The aryl group significantly affects the reactivity and chemical behavior of the hydroxyl group due to the aromaticity of its unsaturated bond system. Some of the electron orbitals of the hydroxyl group’s oxygen atom overlap with the orbitals of the carbon atoms in the aryl ring, which has the effect of reducing the energy required to separate the oxygen-hydrogen bond. This renders phenol and similar compounds sufficiently acidic that they can be deprotonated, or lose one or more hydrogen cations (H⁺), by reaction with a base such as sodium hydroxide. Alkyl alcohols, in comparison, typically require a much stronger base, such as n-butyllithium or sodium hydride, to deprotonate the alcohol hydroxyl functional group.

The hydroxyl group of a phenol allows the oxygen atom to react readily by radical mechanisms as well. In a radical reaction mechanism, the oxygen-hydrogen bond cleaves in such a way that the electrons in the bond are distributed equally, creating two radicals, which are atoms or molecules with unpaired valence electrons. Radicals can undergo a number of different reactions that are not available to ionic species. Two radicals can join together simply by forming a normal covalent bond with their unpaired electrons. The most exotic reaction available to a radical is insertion, in which a radical inserts itself into a bond between two atoms in another molecule. Due to the low energy required to break the aryl oxygen-hydrogen bond, phenols and phenolic compounds produce, or are involved in the production of, a large number of compounds found in nature, including the polymeric material known as lignin. Along with cellulose, lignin is a main structural component of plants. Industrially, lignin decomposition is a source of a number of important chemical products.

Nomenclature of Phenols

The systematic nomenclature of phenols, as devised by the International Union of Pure and Applied Chemistry (IUPAC), is straightforward. Since the structure of the phenol is based on the six-membered benzene ring, with the carbon atom carrying the hydroxyl group designated as the first atom in the ring, naming any simple phenolic compound is a matter of identifying the various other substituent groups and their positions relative to the hydroxyl group. The parent compound is typically identified in the name as phenol rather than benzene. For example, a benzene ring with a hydroxyl functional group on one carbon atom and an isopropyl alkyl substituent on a carbon atom two positions removed can be named 1-hydroxy-3-isopropylbenzene (or 3-isopropylhydroxybenzene), but it can also be named 3-isopropylphenol. Another common naming convention uses the ortho-, meta-, and para- designations to identify the position of a single substituent relative to the primary ring position. Under this naming convention, 3-isopropylphenol can be named m-isopropylphenol, indicating that the isopropyl substituent is in the meta position of the six-membered ring, two carbon atoms away from the hydroxyl functional group. (The ortho position is next to the primary carbon, while the para position is directly opposite it.)

Nomenclature becomes more complicated when there is more than one hydroxyl functional group on the molecule. Because of the six-membered structure of the benzene ring, there can be as many as six hydroxyl functional groups in one molecule. The IUPAC system for naming polyhydroxylated phenols is the simplest method of identifying such a molecule, since each of the basic structures has its own common name. In the IUPAC convention, the substituents are identified first, and then the hydroxyl groups are assigned their corresponding positions such that the substituents occupy the lowest-numbered positions, with one of the hydroxyl groups assigned the position of priority. The full name then consists simply of the substituent groups in alphabetical order with their corresponding position number on the ring. For example, a compound with the structure

would have the systematic name 1,5-dihydroxy-4-isopentyl-2-methoxybenzene, starting with the hydroxyl group at the bottom of the diagram and numbering in a clockwise direction. With such compounds, the simplest way to decide which carbon atom is in the primary position and in which direction the numbering should proceed is to determine which potential combination of position numbers adds up to the lowest sum. In this example, the position numbers used are 1 (hydroxyl group), 2 (methoxy group, H₃CO−), 4 (isopentyl group, (CH₃)₂CH−CH₂−CH₂−), and 5 (second hydroxyl group), which add up to 12. Numbering in the other direction gives the position numbers 1, 3, 4 and 6, which add up to 14. Since 12 is less than 14, that name is the correct one according to the systematic convention. (While the sum would be the same if the carbon with the other hydroxyl group were numbered 1 and the numbering proceeded in a counterclockwise direction, the IUPAC system ranks alkoxy groups, such as methoxy, as higher in priority than alkyl groups, such as isopentyl, meaning that the alkoxy should have the lower number. The priority of various functional groups is a more advanced topic in the nomenclature of organic chemistry.)

It is also possible to name polyphenolic compounds such as this using the common name for the parent structure. In such cases, the parent structure used typically is a matter of choice. All of these names identify the compound unequivocally and are therefore correct in their own right, but only one name is the proper systematic IUPAC name.

Reactions of Phenols

Phenols are reactive compounds that undergo all of the reactions typical of alcohols. One important aspect of the reactivity of phenols is the enhanced acidity of the hydroxyl group. This makes salt formation, in the form of the corresponding phenoxide ion, a much easier process. A simple reaction with hydroxide, for example, yields the phenoxide ion salt, whereas much stronger bases are typically required to produce the corresponding salts in other substances. The phenoxide ion reacts with alkyl halides to form ethers in a reaction known as the Williamson ether synthesis. Several methods have been developed to use the ethers formed by the phenoxide ion as protecting groups, which are used to prevent certain portions of molecules from being affected when the surrounding atoms are taking part in a reaction.

As aryl compounds, phenols are also able to undergo all of the reactions typical of those compounds. Friedel-Crafts alkylations and acylations, diazonium salt reactions, and other electrophilic substitution reactions generally proceed faster and with higher yields than when benzene reacts under the same conditions. The hydroxyl group is termed an activating ortho-para directing group because the products of electrophilic substitution reactions result from substitution in those positions relative to the hydroxyl group. For example, the reaction of phenol with nitric acid produces predominantly p-nitrophenol and o-nitrophenol.

Production of Phenols

Like many chemical compounds, phenols are produced using a wide variety of methods. Phenol itself is a component of coal tar, which is obtained by the destructive distillation of bituminous coal. The process decomposes the molecular structure of the compounds within the coal into many different smaller molecules. In addition to phenols, the distillation of bituminous coal can produce a number of other aryl and polyaryl compounds, such as naphthalene.

Other phenols are synthesized by various reactions typical of benzene and other aromatic compounds. The majority of phenol produced industrially is made using cumene (isopropylbenzene or 2-phenylpropane, C₆H₅CH(CH₃)₂) as the starting material. Reaction of cumene with molecular oxygen (O₂) produces cumene hydroperoxide, the chemical bonds of which are easily cleaved by aqueous acid in a process known as hydrolysis to yield phenol and acetone. Another process, named the Dow process after Canadian-born chemist and industrialist Herbert Dow (1866−1930), reacts chlorobenzene with aqueous sodium hydroxide at a high temperature. Phenol is formed in the reaction as hydroxide displaces chloride by nucleophilic substitution, a process in which one nucleophile, or chemical that tends to react with positively charged or electron-poor species, is substituted for another. A third important industrial method for the production of phenol involves the fusion of sodium benzene sulfonate with an alkali, a high-temperature process in which hydroxide from the molten alkali salt displaces the sulfate ion to yield phenol and sodium sulfate. In laboratory-scale preparations, some phenol compounds can be produced by the hydrolysis of diazonium salts.

PRINCIPAL TERMS

• aromaticity: a characteristic of certain ring-shaped molecules in which alternating double and single bonds are distributed in such a way that all bonds are of equal length and strength, giving the molecule greater stability than would otherwise be expected.
• functional group: a specific group of atoms with a characteristic structure and corresponding chemical behavior within a molecule.
• hydroxyl group: a primary functional group consisting of an oxygen atom covalently bonded to a single hydrogen atom.
• phenyl group: a cyclic functional group with the formula −C₆H₅, similar to a benzene molecule but with one fewer hydrogen atom.
• unsaturated: describes an organic compound in which carbon atoms are attached to other atoms via double or triple bonds, preventing the compound from containing the maximum possible number of hydrogen atoms.

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